Voltage-gated sodium channels play a central role in initiating and propagating action potentials in electrically excitable cells such as neurons and muscle, see for example Yu and Catterall, Genome Biology 4:207 (2003) and references therein. Voltage-gated sodium channels are multimeric complexes characterized by an Alpha-subunit which encompasses an ion-conducting aqueous pore, and is the site of the essential features of the channel, and at least one Beta-subunit that modifies the kinetics and voltage-dependence of the channel gating. These structures are ubiquitous in the central and peripheral nervous system where they play a central role in the initiation and propagation of action potentials, and also in skeletal and cardiac muscle where the action potential triggers cellular contraction. (see Goldin, Ann NY Acad Sci. 30; 868:38-50 (1999).
Sensory neurons are also responsible for conveying information from the periphery e.g. skin, muscle and joints to the central nervous system (spinal cord). Sodium channels are integral to this process as sodium channel activity is required for initiation and propagation of action potentials triggered by noxious stimuli (thermal, mechanical and chemical) activating peripheral nociceptors.
Nine different Alpha-subunits have been identified and characterized in mammalian voltage-gated sodium channels. These structures are designated Nav 1.X sodium channels (X=1 to 9) in accordance with currently accepted nomenclature practice, designating their ion selectivity (Na), the physiological regulator (‘v’, potential, i.e. voltage), and the gene subfamily encoding them (1.), with the number designator X (1 to 9) being assigned for the alpha subunit present in the structure (see Aoldin et al., Neuron, 28:365-368 (2000)). Nav 1.7 voltage-gated sodium ion channels (herein designated “Nav 1.7 channels” in some instances for convenience) are expressed primarily in sensory and sympathetic neurons, are believed to play a role in various maladies, for example, nociception, cough, and itch, and in particular have a central role in inflammatory pain perception, (see Wood et al. J. Neurobiol. 61: pp 55-71 (2004), Nassar et al., Proc. Nat. Acad. Sci. 101(34): pp 12706-12711 (2004), Klinger et. al., Molecular Pain, 8:69 (2012), see Devigili et. al., Pain, 155(9); pp 1702-7 (2014), Lee et. al., Cell, 157:1-12 (2014), Muroi et. al., Lung, 192:15-20 (2014), Muroi et. al., Am J Physiol Regul Integr Comp Physiol 304:R1017-R1023 (2013)).
Loss of function mutations in NaV1.7 lead to Cogenital Insensitivity to Pain (CIP), where patients exhibit a lack of pain sensation for a variety of noxious stimuli (Goldberg et al., Clinical Genetics, 71(4): 311-319 (2007)). Gain of function mutations in NaV1.7, NaV1.8, and NaV1.9 manifest in a variety of pain syndromes where patients experience pain without an external stimulus (Fischer and Waxman, Annals of the New York Academy of Sciences, 1184:196-207 (2010), Faber et al., PNAS 109(47): 19444-19449) (2012), Zhang et al., American Journal of Human Genetics, 93(5):957-966 (2013)).
Accordingly, it is believed that identification and administration of agents which interact to block Nav 1.7 voltage-gated sodium ion channels represents a rational approach which may provide treatment or therapy for disorders involving Nav 1.7 receptors, for example, but not limited to, acute pain, preoperative pain, perioperative pain, post-operative pain, neuropathic pain, cough, or itch disorders, as well as those stemming specifically from dysfunction of Nav 1.7 voltage-gated sodium ion channels, see Clare et al., Drug Discovery Today, 5: pp 506-520 (2000)).
It has been shown in human patients as well as in animal models of neuropathic pain that damage to primary afferent sensory neurons can lead to neuroma formation and spontaneous activity, as well as evoked activity in response to normally innocuous stimuli. [Carter, G. T. and Galer, B. S., Advances in the Management of Neuropathic Pain, Physical Medicine and Rehabilitation Clinics of North America, 2001, 12(2): pp 447 to 459]. Injuries of the peripheral nervous system often result in neuropathic pain persisting long after an initial injury resolves. Examples of neuropathic pain include, for example, post herpetic neuralgia, trigeminal neuralgia, diabetic neuropathy, chronic lower back pain, phantom limb pain, pain resulting from cancer and chemotherapy, chronic pelvic pain, complex regional pain syndrome and related neuralgias. The ectopic activity of normally silent sensory neurons is thought to contribute to the generation and maintenance of neuropathic pain, which is generally assumed to be associated with an increase in sodium channel activity in the injured nerve. [Baker, M. D. and Wood, J. N., Involvement of Na Channels in Pain Pathways, TRENDS is Pharmacological Sciences, 2001, 22(1): pp 27 to 31].
Nociception is essential for survival and often serves a protective function. However, the pain associated with surgical procedures and current therapies to relieve that pain, can delay recovery after surgery and increase the length of hospital stays. As many as 80% of surgical patients experience post-operative pain, which arises as a result of tissue damage, including damage to peripheral nerves and subsequent inflammation). Furthermore, 10-50% of surgical patients will develop chronic pain after surgery often because the nerve damage results in lasting neuropathic pain once the wound has healed (Meissner et al., Current Medical Research and Opinion, 31(11):2131-2143 (2015)).
Cough is one of the most prevalent symptoms for which patients seek the attention of their primary care physicians; chronic cough for example is estimated to affect approximately 40% of the population. The fundamental mechanisms of the cough reflex are complex and involve an array of events initiated by the activation of airway sensory nerves that physically results in a forced expiration of the airways. This protective reflex is necessary to remove foreign material and secretions from the airways, however, chronic, non-protective cough results in a dramatic negative impact on quality of life (see Nasra et. al., Pharmacology & Therapeutics, 124(3):354-375 (2009)).
Cough symptoms can arise from the common cold, allergic and vasomotor rhinitis, acute and chronic bacterial sinusitis, exacerbation of chronic obstructive pulmonary disease, Bordetella pertussis infection, asthma, postnasal-drip syndromes, gastroesophageal reflux disease, eosinophilic and chronic bronchitis, and angiotensin-converting-enzyme inhibitors, cough is categorically described as acute, subacute, or chronic, respectively lasting less than three weeks, three to eight weeks, and more than eight weeks in duration (see Irwin et. al., The New England Journal of Medicine, 343(23):1715-1721 (2000)).
Current standard of care for the treatment of cough consists of centrally and peripherally acting suppressants such as opioids and local anesthetics respectively, both of which are dose-limited by side-effects (see Cox et. al., Best Practice & Research Clinical Anaesthesiology, 117(1):111-136 (2003) and Benyamin et. al., Pain Physician, 11:S105-S120 (2008)). Opioids primarily act on μ-opioid receptors of the central nervous system, and in some reports, also on peripheral afferents of the cough reflex arc—they exhibit varied degrees of efficacy and are limited by side-effects such as sedation, physical dependence, and gastrointestinal problems; morphine has shown to be an effective treatment for chronic cough (see Morice et. al., Am J Respir Crit Care Med 175:312-315 (2007) and Takahama et. al., Cough 3:8 (2007)), but is generally restricted to patients with terminal illness such as lung cancer. Codeine, found in some cough syrups, and also administered systemically, was found no more effective than placebo (see Smith et. al., Journal of Allergy and Clinical Immunology, 117:831-835 (2006). Local anesthetics act peripherally by reducing the generation of action potentials in sensory nerves of the airway as a result of non-selectively inhibiting all voltage gated sodium channel subtypes and have demonstrated varied degrees of efficacy in treating cough. These compounds are often found in over-the-counter lozenges and have been shown to relieve cough when administered via nebulisation (see Nasra et. al., Pharmacology & Therapeutics, 124(3):354-375 (2009) and Hansson et. al., Thorax, 49(11):1166-1168 (1994)). However, in a study with chronic obstructive pulmonary disease patients, lidocaine was not effective (see Chong et. al., Emerg Med J, 22(6):429-32 (2005)).
In pre-clinical animals, Nav 1.7, Nav 1.8 Nav1.8, and NaV1.9 were determined to be the primary voltage-gated sodium channels expressed in the afferent nerves of the respiratory tract (see Muroi et. al., Lung, 192:15-20 (2014)) and in animal models of cough, suppression of NaV1.7 function resulted in a marked decrease in number of coughs (see Muroi et. al., Am J Physiol Regul integr Comp Physiol, 304:R1017-R0123 (2013)), thus, combined with previous evidence that local anesthetics can be effective antitussive agents, the targeted blockade of NaV1.7 channels is believed to represent a rational approach for the treatment of cough with a preferential side-effect profile as compared to local anesthetics. Local anesthetics undesirably inhibit all voltage gated sodium channels, such as NaV1.5 channels which are found in heart muscle (see Rook et. al., Cardiovascular Research 93:12-23 (2012)).
Pruritus, also commonly known as itch, affects approximately 4% of the global population (see Flaxman et. al., Lancet, 380:2163-2196 (2012)) is “an unpleasant sensation that elicits the desire or reflex to scratch” and is regarded as closely related to pain. Theories on the origin of itch implicate the subtle, low-frequency activation of nociceptors (pain-sensing neurons), however, it has been described that some afferents preferentially respond to histamine, which induces itch (see Schmelz et. al., J Neuroscience, 17(20):8003-8008 (1997)). At the same time, it has been found that histamine-responding neurons also respond to capsaicin which produces pain (see McMahon et. al., Trends. Neurosci., 15:497-501 (1992)). Members of the transient receptor potential (TRP) family, and nerve growth factor (NGF) are both known to play a role in itch and pain, and clinically, both maladies are treated with therapeutic agents such as gabapentin and antidepressants—as such, it continues to be accepted that the underlying mechanisms of pain and itch are highly interwoven and complex, and distinguishing pan-selective or itch-selective pathways remains ambiguous (see Ikoma et. al., Nature Reviews Neuroscience, 7:535-547 (2006)).
Itch, both chronic and acute, can arise from many different insults and diseases and may be classified as dermal or pruriceptive, neurogenic, neuropathic, or psychogenic: itch can arise from both systemic disorders, skin disorders, as well as physical or chemical insult to the dermis. Pathologically, conditions such as dry skin, eczema, psoriasis, varicella zoster, urticaria, scabies, renal failure, cirrhosis, lymphoma, iron deficiency, diabetes, menopause, polycythemia, uremia, and hyperthyroidism can cause itch, as can diseases of the nervous system such as tumors, multiple sclerosis, peripheral neuropathy, nerve compression, and delusions related to obsessive-compulsive disorders. In skin, pruritogens are released from keratinocytes, lymphocytes, mast cells, and eosinophils during inflammation. These molecules act directly on free nerve endings to induce itch; medicines such as opioids and chloroquine can also trigger itch (see Ikoma et. al., Nature Reviews Neuroscience, 7:535-547 (2006)). Itching following burn is also an extremely serious clinical problem as it hampers the healing process, results in permanent scaring, and negatively impacts quality of life (see Loey et. al., British Journal of Dermatology, 158:95-100 (2008)).
Gain of function mutations of NaV1.7 have been found in approximately 28% of patients with idiopathic small fiber neuropathy (I-SFN); these mutations were found to render dorsal root ganglia neurons hyperexcitable, reducing the threshold of activation and increasing the frequency of evoked firing (see Waxman et. al., Neurology, 78(21):1635-1643 (2012)). Severe, uncrontrollable itch has also been genetically linked to a gain-of-function mutation (I739V) in the sodium channel NaV1.7 in man (see Devigili et. al., Pain, 155(9); pp 1702-7 (2014)). Additionally, the sea-anemone toxin ATX-II has been found to elicit pain and itch in human volunteers after intradermal injection on the forearm; electrophysiology studies revealed that ATX-II enhanced NaV1.7 and NaV1.6 resurgent currents (see Klinger et. al., Molecular Pain, 8:69 (2012)). It has been demonstrated in animal models that selective blockade of NaV1.7 channels can effectively suppress both inflammatory and neuropathic pain, as well as acute and chronic itch, thus blockade of NaV1.7 channels is believed to represent a rational approach to treatment of pain and itch disorders (see Lee et. al., Cell, 157:1-12 (2014)).
Because voltage gated sodium ion channels are ubiquitous in the central and peripheral nervous system, as well as in both cardiac and skeletal muscle, and conservation of structures in the various Alpha-subunits characterizing voltage-gated sodium ion channels implicates the potential for producing serious side effects when utilizing therapeutic agents having a mechanism of action that target inhibition of voltage-gated sodium ion channels, for example, therapeutic agents suitable for use in addressing nociception, cough, or itch disorders, requires therapeutic agents having specificity in their action, for example, discriminating between action upon Nav 1.5 sodium ion channels, thought to be important in regulation of cardiac function and action upon Nav 1.7 sodium ion channels, thought to be central in inflammatory nociception, cough, or itch and disorders arising from dysfunctional Nav 1.7 sodium ion channels.
There remains a need for additional compounds having high potency for inhibiting Nav 1.7 sodium ion channels and selective activity for Nav 1.7 sodium ion channels providing structural variety to facilitate rational development of therapeutic agents for use as a selective Nav 1.7 sodium ion channel inhibitor.